The primary objective of this study is to determine if BG9588 can decrease renal inflammation without a worsening of renal function in subjects with WHO Class III or IV SLE GN. The primary endpoint is the proportion of subjects with a reduction in renal inflammation sustained for greater than and equal to 4 weeks during the fifth and sixth cycles of treatment as measured by the following: a greater than and equal 50% reduction from baseline proteinuria and to <3 g/day and a greater than and equal to 50% reduction from the baseline hematuria and to <20 rbc.hpf, without worsening of renal function during the same 4-week period. Worsening of renal function is defined as the following: a greater and equal 33% increase from baseline serum creatinine and a greater than and equal to 25% decrease from baseline GFR30. These criteria must not be explainable by other events, e.g., dehydration, renal calculus, renal infection or concomitant medication therapy changes